Yes, the old version files are indeed stored in your account. By default, these files are stored in hidden folders. It is not recommended to change any history files.
You can logon www.drivehq.com, go to My Account page, then click on Account Options, check the checkbox "Show hidden folders and old version folders".
After you save your changes, the history folders will be displayed on DriveHQ.com website and DriveHQ FileManager client software. In general, old version files are saved in a hidden folder that's in the same parent folder of the current file. The hidden folder name is like: \FILENAME~\
Because multiple versions of the same file might be created, the file name is also changed to FILE_ID.EXT, here EXT is the original file extension.
When you change or update a file on DriveHQ, the file name is kept the same, but the internal file ID changes. It is not possible to predict the new file IDs. You can list the hidden folder and pick the right version based on file modify time.